In existing air-conditioning apparatuses such as multi-air-conditioning apparatuses installed in buildings, for example, outdoor units that are installed outside the buildings and serve as heat source units and indoor units installed inside the buildings are connected by pipes to form refrigerant circuits in which refrigerants circulate. Air is heated or cooled by utilizing heat transfer or heat removal as the refrigerants travel through the refrigerant circuits, to heat or cool the air-conditioned spaces.
When a heating operation is performed at an outside air temperature below approximately −10 degrees C. by such a multi-air-conditioning apparatus installed in a building as described above, the low-temperature outside air and the refrigerant exchange heat with each other. Thus, the evaporating temperature of the refrigerant decreases, and its evaporating pressure decreases accordingly.
Consequently, the density of a refrigerant drawn by suction into a compressor decreases and the refrigerant flow rate, in turn, decreases, resulting in an insufficient heating capacity of the air-conditioning apparatus. In addition, as the density of a refrigerant drawn by suction into the compressor is low, the compression ratio is high, causing an excessive increase in the temperature of the refrigerant discharged from the compressor. Thus, problems such as deterioration of refrigerating machine oil and damage to the compressor occur.
In order to address the problems described above, an air-conditioning apparatus has been proposed (see, for example, Patent Literature 1) which is configured to inject a two-phase refrigerant into a region where an intermediate pressure is obtained in the compression process of the compressor to improve the density of a refrigerant to be compressed and thereby increase the refrigerant flow rate so that a sufficient heating capacity can be achieved when the outside air temperature is low to reduce the discharge temperature of the compressor.
The technique described in Patent Literature 1 utilizes the fact that when the saturation temperature of a high-pressure refrigerant supplied to a load side heat exchanger becomes equal to or higher than the temperature of the indoor air, heat is transferred from the high-pressure gas refrigerant to the indoor air so that the refrigerant liquefies into a two-phase refrigerant. In this case, the two-phase refrigerant is injected into a region where an intermediate pressure is obtained in the compression process of the compressor to reduce the temperature of the refrigerant discharged from the compressor.